midx: stop duplicating info redundant with its owning source

midx: stop duplicating info redundant with its owning source

Multi-pack indices store some information that is redundant with their
owning source:

  - The locality bit that tracks whether the source is the primary
    object source or an alternate.

  - The object directory path the multi-pack index is located in.

  - The pointer to the owning parent directory.

All of this information is already contained in `struct odb_source`. So
now that we always have that struct available when loading a multi-pack
index we have it readily accessible.

Drop the redundant information and instead store a pointer to the object
source.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>